1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus including the developing device.
2. Description of the Related Art
The following type of developing device used in image forming apparatuses, such as laser printers, has been developed. That is, the developing device includes a toner supplying roller that supplies toner (developer) to a developing roller (a developer bearing member) and removes the toner carried by the developing roller. The toner supplying roller is mainly used to prevent solid image follow-up failure and ghosting. As used herein, the term “solid image follow-up failure” refers to a phenomenon that when 100% solid image is drawn as an entire image, the density of part of the image in the trailing edge area decreases. In addition, the term “ghosting” refers to a phenomenon that when, for example, a halftone image or a solid white image is formed after a solid image having a high density is formed, part of the solid image is left on the halftone image or the solid white image.
In recent years, a developing device that does not have the above-described toner supplying roller has been developed in order to reduce the size and cost of the developing device. In such a case, some other way needs to be found to prevent the occurrence of the solid image follow-up failure and ghosting.
Japanese Patent No. 3272056 describes a developing device without a toner supplying roller. The developing device includes a developing roller (a developer bearing member) having a surface in which dielectric portions and conductive portions are regularly or irregularly arranged. In such a configuration, a development blade (a regulating member) slides on the dielectric portions of the surface of the developing roller directly or via toner so as to charge the dielectric portions. Thus, a minute closed electric field (hereinafter referred to as a “microfield”) is formed on the border between the dielectric portion and the conductive portion. Upon receiving a gradient force generated by the microfield, toner particles are attracted to the surface of the developing roller and, thus, are carried by the surface of the developing roller.
Alternatively, Japanese Patent Laid-Open No. 4-218079 describes a developing roller that allows a plurality of types of substance having different chargeabilities to be regularly or irregularly exposed from the developing roller. Among the substances, at least two substances are charged by a charging member to form a plurality of microfields in the vicinity of the developing roller. Upon receiving a gradient force generated by the microfields, toner particles are attracted to the surface of the developing roller and, thus, are carried by the surface of the developing roller.
Still alternatively, Japanese Patent Laid-Open No. 4-127177 describes the following configuration. That is, a triboelectric charging roller is disposed downstream of a developing portion, which is a contact portion between a developing roller and an image bearing member, in the rotational direction of the developing roller. The triboelectric charging roller slides on dielectric portions of the developing roller to charge the dielectric portions. By charging the dielectric portions in this manner, a microfield is formed on the border between the dielectric portion and a conductive portion. Upon receiving a gradient force generated by the microfield, toner particles are attracted to the surface of the developing roller and, thus, are carried by the surface of the developing roller.
The developing device described in Japanese Patent No. 3272056 is configured so that if the toner is charged to a negative polarity, a triboelectric series of (−) the toner<the development blade<the dielectric portion (+) is given. In such a configuration, since the toner carried by the dielectric portions is electrostatically and firmly attracted to the dielectric portions, it is difficult for the development blade to regulate the toner. Accordingly, when a solid white image is formed, the amount of toner coat on the developing roller is sometimes larger than when a solid image is formed. Thus, the difference between the amounts of toner coat may cause ghosting in an image.
In addition, although the development blade can control the amount of toner coat, the development blade cannot remove toner on the developing roller, unlike a toner supplying roller. Accordingly, if images having a low coverage rate are continuously output, toner may be fusion bonded to the developing roller, which causes an image defect. To prevent such an image defect, the lifetime of the developing device needs to be set to be short.
In addition, like the developing devices described in Japanese Patent Laid-Open No. 4-218079 and Japanese Patent Laid-Open No. 4-127177, in the configuration including a toner supplying roller and a triboelectric charging roller, toner on the developing roller is strongly pressed against the developing roller or is stripped off from the developing roller. Accordingly, the toner may be fusion bonded to the developing roller, or the quality of the toner may be deteriorated. As a result, an image defect may occur. To prevent such an image defect, the lifetime of the developing device needs to be set to be short.